The invention relates to a pulse generator for spark-erosive metal working.
The tendency in spark erosion technology is towards ever higher working or machining rates. Therefore pulse generators are required, which are able to supply high energy levels with a high repetition rate to the spark gap. High energy pulses with a clearly defined, very short pulse time are particularly required in the field of spark-erosive wire cutting and drilling.
German Pat. No. 27 35 403=U.S. Pat. No. 4,163,887 discloses a pulse generator, which supplies a pulse current up to 500 A in the 1 microsecond range. However, pulse currents up to 1000 A at approximately 2 microseconds are presently required. The aforementioned generator type is too large and is also not economical (due to its poor efficiency of typically 10%) when such high currents are required.
German Pat. No. 34 19 945 discloses a generator with an efficiency of better than 60%. As this generator is designed for all spark-erosive working types, it becomes technically too complex for the applications described above. In addition, the switching devices used therein must switch out the full peak current, which leads to unnecessary switching losses.
German Pat. No. 24 42 734 discloses a generator, in which by means of an electronically controlled charging regulator a storage capacitor is linearly charged. On reaching the charging voltage, the capacitor is discharged via a switching device and an isolation transformer into the spark gap. Generators of this type are technically complex and have a limited range of control because the discharge must always be initiated when the storage capacitor is completely charged, which in certain circumstances coincides with unfavorable conditions in the spark gap.
Another type of generator is described in U.S. Pat. No. 3,485,987, where the charging current for a storage capacitor is coupled into the erosion circuit across an isolation transformer. The storage capacitor is alternately charged by means of a first switching device and discharged by means of a second switching device. However, this arrangement has the serious disadvantage that in the case of a "no-load pulse", i.e. a pulse which does not ignite a spark, the storage capacitor is not recharged so that the circuit is consequently only operative for the next but one pulse. There are also reactions between the two switching devices across the isolation transformer, which can only be controlled by high protective wiring expenditure.
Various other resonant circuit generator circuits are known, which have the advantage that the switching devices commutate dead or with only a limited current.
An important disadvantage of generators with electronically controlled charging regulators is that they are subject to deterioration of overall efficiency. Resonant circuit generators without controlled charging regulators suffer from the disadvantage of difficultly controllable current pulses (across the spark gap), because the charging state of the storage capacitor is greatly influenced by the preceding pulse.